1. Field of the Invention
This invention relates to an apparatus for detecting a condition of burning in an internal combustion engine.
2. Description of the Prior Art
Japanese published unexamined patent application 61-57830 discloses a method and a device for deciding on abnormal combustion. According to Japanese application 61-57830, when an air-fuel mixture gas in a combustion engine is fired by the spark discharge of an ignition plug, a voltage for measurement is applied to the ignition plug and an ion current flows between electrodes of the ignition plug. The ion current has various frequency components. In Japanese application 61-57830, the ion current causes a resistance to generate a terminal voltage, which is amplified for a necessary time after the discharge. A knock component and a natural oscillation component are extracted from the terminal voltage by filters respectively. Outputs of the filters are integrated, and a divider calculates the ratio between the integration results. In Japanese application 61-57830, the output voltage of the divider is compared with a voltage corresponding to a predetermined knocking intensity limit to decide on abnormal combustion.
Japanese published unexamined patent application 50-94330 discloses an apparatus for detecting a misfire in an internal combustion engine. In the apparatus of Japanese application 50-94330, detection is made as to an ignition-indicating signal and an ion current signal outputted from an ignition plug. The ion current signal contains a burning signal which is slightly delayed from the ignition-indicating signal. The apparatus of Japanese application 50-94330 includes a comparator which serves to compare the ion current signal with an ignition command signal synchronous with a high tension voltage applied to the ignition plug. Only when both the ignition command signal and the burning signal are simultaneously effective, the comparator outputs an active signal. The apparatus of Japanese application 50-94330 includes a deciding section which determines whether or not an air-fuel mixture in a related engine cylinder has been successfully ignited in response to the output signal of the comparator.
In the apparatus of Japanese application 50-94330, an ion gap has a resistance depending on ion conditions in the related engine cylinder. In addition, an ion current occurs which depends on the resistance of the ion gap. The apparatus of Japanese application 50-94330 includes a filter for removing noise components from the ion current. The output signal of the filter is shaped into a pulse signal constituting the ion current signal.
Generally, magnetic energy tends to remain in an ignition coil after the end of discharge across an ignition plug. This residual magnetic energy causes noise superimposed on an ion current signal outputted from the ignition plug. Both Japanese application 61-57830 and Japanese application 50-94330 fail to teach a countermeasure for such noise.